This invention relates to an AC electrolytic capacitor and more particularly to an electrolyte for an aluminum AC capacitor that has good temperature stability and long life.
Aluminum electrolytic capacitors generally have at least a pair of aluminum foil electrodes, one of these having a dielectric oxide formed on its surface, spacer material, a container with an end seal, and an electrolyte. The foil electrodes may be etched to increase the surface area of the foil. The foils are usually separated by dielectric spacer materials such as paper, plastic film, or combinations of these. The foils and spacers may be convolutely wound into a roll that is impregnated with electrolyte before being placed in the container and sealed.
Aluminum electrolytic capacitor electrolytes conventionally have an ionizable compound dissolved in a solvent, the commonest being the borate-glycol system. Other electrolytes are formulated with glycol ethers, organic alcohols, and amides as solvents with organic acids, at least partially neutralized, as ionogens.
Those aluminum electrolytic capacitors that find use as AC motor-start capacitors are subjected to elevated temperatures of 100.degree. C. or more and to heavy-duty cycling, e.g., frequent starting with high voltage and current conditions. A frequent failure mode of this type of service is that of the electrolytic breaking down under the high temperature conditions. As is pointed out by Anderson in U.S. Pat. No. 4,024,442, glycol ether-based electrolytes have a boiling point that is too low for most AC motor-start applications and the use of such electrolytes may cause catastrophic failure of the capacitor at elevated temperatures. Also, glycol ether-based electrolytes tend to dissolve the potting compounds used to secure the foil roll to the container to prevent movement relative to the container. Since these ethers are good solvents, they may also attack the container and/or seal itself, particularly if either is a phenolic material. Ethylene glycol is being used as a solvent for AC electrolytic capacitors electrolytes now for these and other reasons.